As one of the important elements in the research and development of nuclear energy, thorium element itself does not decay easily, but may be converted to fissile U-233 after absorbing a neutron, and thus is a very promising nuclear fuel. Thorium is abundant in the earth's crust with a content of about 6 ppm, which is about 3 times of that of uranium. In china, thorium is mainly associated in rare earth ores and has a total reserve of about 280-300 thousand tons. Currently in China, however, the utilization rate of thorium is almost zero. In the production of rare earth (RE), thorium is discharged to the tailing dam or the neutralized slag of rare earth, which results in a large radioactive safety risk and a huge resource waste. Once there is breakthrough in the research of thorium nuclear energy, huge benefit will be brought and the dependency on oil will be largely released. Thus, it is of great interest to recover and purify thorium.
Chinese patent application No. 98122348.6 and 02123913.4 disclosed two processes of recovering thorium during extraction and separation of rare earth from rare earth ores (Baotou mixed bastnasite-monazite ore and Sichuan bastnasite), in which a primary amine (R—NH2) is used as an extractant and thorium was recovered and enriched from about 0.2% (in refined rare earth ore) to 95-99% with a thorium recovery of 95%. Gupta in Indian studied recovering and separating thorium, uranium and rare earth from monazite with Cyanex 923 (Journal of Radioanalytical and Nuclear Chemistry, 251 (2002) 451-456), and found that thorium and uranium could be extracted in a medium of 5.0 mol/l nitric acid, and then thorium was back-extracted by 2.0 mol/l hydrochloric acid and uranium was stripped by 0.5 mol/l sulfuric acid. However, the process concerned these results are not reported.
To meet the requirement for the research and power generation of thorium nuclear energy, thorium used as a nuclear fuel must be at nuclear purity level, so the thorium enrichments from rare earth ores or other resources must be purified further. In an early process, tributyl phosphate (TBP) is used as an extractant to extract and purify thorium from the thorium hydroxide enrichments obtained from the processing of monazite (W U Huawu, Nuclear Fuel Chemical Technology, Atomic Energy Press, 1989, p. 164). In this process, a TBP extraction system was adopted to separate uranium and purify thorium, using a thorium hydroxide enrichment obtained from the processing of monazite as the feed. FIG. 1 shows the flow chart of the process, in which Ss represents the acidized organic phase, F represents the feed, W represents the scrubbing solution, St represents the stripping solution, Ac represents the acid for acidizing, R represents the extraction raffinate, and the other process parameters are shown in table 1.
TABLE 1the process parameters for purifying thorium with TBPmaterialsFlownameabbreviationcompositionratioacidized organic phaseSs140 vol. % TBP-paraffin4acidized organic phaseSs240 vol. % TBP-paraffin0.8feedF0.76 mol/l Th(NO3)4;1which contains 4 mol/l HNO3 and additional U,RE and Fescrubbing solutionW4 mol/l HNO30.8stripping slotuionSt10.1 mol/l HNO34stripping solutionSt25% Na2CO33.2acid for acidizingAc4 mol/l HNO3/
However, the purification of thorium with TBP needs to be performed in a high concentration nitric acid medium, which results in a large consummation of the acid and an accelerated decomposition of TBP. In addition, TBP has a low molecular weight with a relative high solubility in water (0.64 g/L at 25° C.), which causes the running off of TBP and thus an increased cost.